High sensitivity image-based reflectometry

ABSTRACT

Methods for performing imaging reflectometry measurements include determining a representative reflectance intensity value using multiple images of a measurement area that includes a particular structure and/or using a plurality of pixels each associated with the particular structure within the measurement area. A parameter associated with the particular structure is determined using the representative reflectance intensity value.

FIELD

Embodiments described herein relate generally to imaging reflectometers,and more particularly, to methods for improving sensitivity ofimage-based reflectometry measurements.

BACKGROUND

Imaging reflectometry can be used to measure critical dimensions (CDs)of small features and thicknesses of films. Systems that perform thesemeasurements typically utilize either a spot scanning mechanism or aline scanning mechanism. For a spot scanning mechanism, a spectrum ofeach spot is recorded by a spectrometer that generally includes agrating or prism to distribute the spectrum onto a line sensor. For aline scanning mechanism, each row of an area sensor records an image ofa scanning line and each column records a spectrum. These mechanismsprovide flexibility for handling large samples or samples with largemeasurement fields.

Imaging reflectometry systems and methods that provide improvedmeasurement sensitivity are desired.

SUMMARY

Embodiments described herein provide improved sensitivity of image-basedreflectometry measurements. Image-based reflectometry measurements canbe used to identify different characteristics or features of a sampledepending on the contrast generated. The ability of image-basedreflectometry measurements to distinguish between similarcharacteristics or features depends on a signal to noise ratio.Increasing the number of electrons in each pixel of an image sensor canincrease the signal, but each pixel has a full well capacity that limitsthe amount of charge a pixel can hold before reaching saturation. Toovercome this limitation, some embodiments described herein increase areflectivity signal by combining measurements from pixels associatedwith a particular structure on a sample and/or by combiningreflectometry measurements from pixels associated with a particularstructure in multiple images. This effectively increases a number ofelectrons without exceeding full well capacity.

In accordance with a particular embodiment, for example, a method forperforming imaging reflectometry measurements includes illuminating ameasurement area on a sample using an input beam; receiving portions ofthe input beam reflected from the sample at an imaging sensor; obtainingmultiple images of the measurement area using the portions of the inputbeam reflected from the sample and received at the imaging sensor, eachof the multiple images comprising a plurality of pixels, whereincorresponding pixels include a single pixel from each of the multipleimages that is associated with approximately a same part of themeasurement area in each of the multiple images; determining areflectance intensity value for each of the plurality of pixels in eachof the multiple images; determining a representative reflectanceintensity value for each of the corresponding pixels based on thereflectance intensity value of each pixel of the corresponding pixels;and determining a parameter associated with a structure on a surface ofthe sample within the measurement area based at least in part on therepresentative reflectance intensity value of each of the correspondingpixels that are associated with the structure in the multiple images.

In an embodiment, the representative reflectance intensity value foreach of the corresponding pixels is an average based on the reflectanceintensity value of each pixel of the corresponding pixels.

In another embodiment, the method also includes determining a compositeimage of the measurement area, wherein each pixel of the composite imageis determined based on the representative reflectance intensity valuefor each of the corresponding pixels.

In another embodiment, the parameter associated with the structure isfilm thickness or critical dimension.

In some embodiments, each of the multiple images are obtained usingapproximately a same integration length of time, or each of the multipleimages are obtained using a different integration length of time.

In another embodiment, the portions of the input beam are reflected fromthe surface of the sample.

In yet another embodiment, the structure comprises a plurality ofstructures.

In accordance with another embodiment, a method for performing imagingreflectometry measurements includes illuminating a measurement area on asample using an input beam, the measurement area including firstsub-areas that each include similar structures on a surface of thesample, and second sub-areas that each include similar structures on asurface of the sample that are different than the similar structures inthe first sub-areas; receiving portions of the input beam reflected fromthe sample at an imaging sensor; obtaining an image of the measurementarea using the imaging sensor, the image comprising a plurality ofpixels; identifying a first portion of the plurality of pixels that areassociated with the first sub-areas and a second portion of theplurality of pixels that are associated with the second sub-areas;determining a first reflectance intensity value for each pixel of thefirst portion of the plurality of pixels that are associated with thefirst sub-areas; determining a first representative reflectanceintensity value based on the first reflectance intensity value of eachpixel in the first portion of the plurality of pixels that areassociated with the first sub-areas; and determining a parameterassociated with the similar structures in the first sub-areas based atleast in part on the first representative reflectance intensity value.

In an embodiment, at least some of the first sub-areas are notimmediately adjacent to others of the first sub-areas within themeasurement area.

In another embodiment, the similar structures in the first sub-areashave a similar surface pattern, and the similar structures in the secondsub-areas have a different surface pattern.

In another embodiment, the method also includes determining a secondreflectance intensity value for each pixel of the second portion of theplurality of pixels that are associated with the second sub-areas;determining a second representative reflectance intensity value based onthe second reflectance intensity value of each pixel in the secondportion of the plurality of pixels that are associated with the secondsub-areas; and determining a parameter associated with the similarstructures in the second sub-areas based on the second representativereflectance intensity value.

In yet another embodiment, the similar structures in the first sub-areaseach comprise a plurality of structures.

Further aspects, advantages, and features are apparent from the claims,description, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments described herein, both as to organization andmethod of operation, together with features and advantages thereof, canbest be understood by reference to the following detailed descriptionand accompanying drawings, in which:

FIG. 1 is a simplified cross-sectional view of an imaging reflectometer.

FIG. 2 is a simplified cross-sectional view of a multi-wavelength lightsource.

FIG. 3 is a simplified cross-sectional view of an imaging reflectometersystem configured to provide area reflectometry measurements and spotreflectometry measurements.

FIG. 4 is a flowchart outlining a method for measuring reflectivity of asample.

FIG. 5A is an image showing a measurement area on a sample that includesdifferent types of structures, and FIG. 5B is a plot illustratingmeasurement noise from a single pixel and measurement noise frommultiple pixels in accordance with an embodiment.

FIG. 6 shows multiple images that each include a measurement area inaccordance with an embodiment.

FIG. 7 is an image highlighting similar structures within a measurementarea in accordance with an embodiment.

FIG. 8-9 are flowcharts illustrating methods for performing imagingreflectometry measurements in accordance with some embodiments.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the embodimentsdescribed herein. However, it should be understood that the variousembodiments can be practiced without these specific details. In otherinstances, well-known methods, procedures, and components have not beendescribed in detail so as not to obscure the described features.

Reference will be made in detail to the various embodiments, one or moreexamples of which are illustrated in the figures. Each example isprovided by way of explanation and is not meant as a limitation.Further, features illustrated or described as part of one embodiment canbe used on or in conjunction with other embodiments to yield yet furtherembodiments. The description is intended to include these modificationsand variations.

A “specimen” or “sample” as referred to herein, includes, but is notlimited to, a semiconductor wafer, a semiconductor work piece, aphotolithography mask, and other work pieces such as a memory disk andthe like. According to some embodiments, which can be combined withother embodiments described herein, the systems and methods areconfigured for or are applied to reflectometry applications.

Embodiments described herein relate generally to improving sensitivityof imaging reflectometry measurements. In some embodiments, areflectivity signal is increased by combining measurements from pixelsassociated with a particular structure on a sample. The particularstructure may include multiple structures on the sample that may or maynot be adjacent to one another. In other embodiments, the reflectivitysignal is increased by combining reflectometry measurements from pixelsassociated with a particular structure in multiple images.

FIG. 1 is a simplified cross-sectional view of an imaging reflectometer100 in accordance with an embodiment. The imaging reflectometer 100shown in this example can be used to implement the methods describedherein. However, the imaging reflectometer 100 is shown merely as anexample and other imaging reflectometers can perform the describedmethods. Merely by way of example, imaging reflectometers configured toobtain images of a part of a sample, rather than an entire sample,and/or imaging reflectometers using single or multi-wavelength lightsources, may be used to perform the methods.

In the example of FIG. 1, light from a source module 102 is relayedthrough a light guide 106 to a homogenizer 108. Light 116 from thehomogenizer 108 passes through illumination pupil 114 and is directedtoward beam splitter 120. A portion 138 of the light 116 is reflected bythe beam splitter 120 toward a reference sensor 134, and a portion 122of the light passes through the beam splitter 120 and continues along anoptical path toward a sample 130.

The portion 122 of the light 116 that passes through the beam splitter120 is imaged by a large field lens 126 onto the sample 130. Lightreflected from the sample 130 is directed through at least a portion ofthe lens 126 and reflected by the beam splitter 120 toward an imagingsensor 158.

The imaging reflectometer 100 may include a number of other lenses(e.g., 110, 112, 118, 136) that shape and/or direct the light along theoptical paths to illuminate the sample 130, illuminate the referencesensor 134, direct the light to other lenses (e.g., 120, 140, 144), anddirect the light reflected from the sample to the imaging sensor 158. Insome embodiments, for example, the light may pass through one or morepolarizers (e.g., polarizers 110, 154). The one or more polarizers canbe inserted in an illumination and/or an imaging path to provideenhanced sensitivity for the dimension change of pattern structuresand/or film thickness on the sample 130 when patterns are not circularlysymmetric. Wave plates can also be inserted to alter the phase of thepolarized light. The wave plates and/or polarizers can be at fixedangles to provide polarized reflectometry measurements or can berotating to provide ellipsometry measurements. It should be appreciatedthat imaging reflectometers in accordance with the embodiments describedherein may not include all the optical elements shown in the example ofFIG. 1 and/or may include other optical elements that are not includedin this example.

The source module 102 in this example provides a multi-wavelength lightsource that may sequentially generate different light beams each havinga narrow wavelength range. In some embodiments, the multi-wavelengthlight source is provided by a plurality of light sources that can beactivated individually. Each of the light sources generates a lightbeam, and at least some of the light beams have different nominalwavelengths.

In other embodiments, the multi-wavelength light source is provided byadjusting the source power to the source module 102 to generate lightbeams having different nominal wavelengths. The power of each wavelengthcan be independently controlled to optimize the dynamic range ofmeasured reflectance at each wavelength.

In yet other embodiments, the multi-wavelength light source is providedby a broadband light source and a set of band pass filters. Thebroadband light source may be used with the band pass filters togenerate the light beams at selected nominal wavelengths.

In yet other embodiments, the source module 102 may include both aplurality of light sources, a broadband light source, and a set of bandpass filters.

In an embodiment, the large field lens 126 has a measurement field size(or illumination area) that is slightly greater than a size of thesample 130 so that full-sample images can be acquired by the imagingsensor 158 without scanning the light or moving a stage 132. Forexample, the large field lens 126 may have a measurement field size of300 mm or more for measuring a semiconductor wafer having a 300 mmdiameter. The large field lens 126 may be a telecentric lens so thatlight rays traveling from the large field lens 126 to the sample 130 areapproximately parallel to an optical axis that is substantiallyperpendicular to a surface of the sample 130. This providessubstantially normal illumination over the entire sample 130 or acrossan entire measurement area. This can reduce measurement error since theillumination angles are approximately the same. Telecentric imagingallows the light reflected at substantially the same angle across theentire field to reach an imaging sensor. In an embodiment, for example,the light illuminating the sample 130 may have a telecentricity error ofless than 0.3 degrees over a wavelength range of about 350 nm to about1200 nm, and in some embodiments, a telecentricity error of less than 1%over a wavelength range of about 350 nm to about 1100 nm. As usedherein, the telecentricity error is a measure of angular deviation of alight ray incident and reflected from the wafer surface from normal (orfrom the optical axis).

In some embodiments, the large field lens 126 has a field size that issmaller than a diameter of the sample 130. In this case, an area (ormeasurement area) is imaged and the optics and/or the stage 132 may bemoved and/or the optical module may be scanned to image adjacent fields.Depending on the application, a measurement area may be approximatelythe same size as a die or stepper field. Adjacent images may be stitchedusing known techniques to provide multi-field or full-sample images.

The imaging sensor 158 may be an area imaging sensor that includes oneor more digital cameras for capturing the light 142 that is reflectedfrom the sample 130 and passes through imaging pupil 150. The imagingsensor 158 provides an image of the sample 130 based on the receivedlight 142. The imaging sensor 158 may include a single camera in someembodiments that is configured to image the entire surface of the sample130. The imaging sensor 158 may include multiple cameras in otherembodiments that each image adjacent or slightly overlapping fields (ormeasurement areas) on the sample 130. Adjacent images may be stitchedtogether using known techniques. Image resolution may be increased byusing a higher resolution imaging sensor or using multiple imagingsensors that each image a smaller field.

The imaging reflectometer 100 includes an illumination path thatprovides light to the sample 130 and an imaging path that provides lightto the imaging sensor 158. This allows independent control of anillumination numerical aperture (NA) and an imaging NA. Merely by way ofexample, if the imaging sensor 158 has an array size of 5120 pixels by5120 pixels and the imaging NA is about 0.004, the pixel size on thesample 130 is about 60 μm for a 300 mm wafer, which has a Rayleighresolution of about 55 μm at a wavelength of 365 nm and a Rayleighresolution of about 153 μm at a wavelength of 1 μm. Generally, theillumination NA is greater than the imaging NA to correct residualchromatic telecentric errors and to provide tolerance to tilt and bow ofthe sample 130. In some embodiments, the illumination NA may range fromabout 0.005 to about 0.5, and the imaging NA may range from about 0.003to about 0.2.

The reference sensor 134 may include one or more digital cameras forcapturing the light 138 that is reflected from the beam splitter 120.The reference sensor 134 may have a lower resolution than the imagingsensor 158. The reference sensor 134 may be used to monitor uniformityand stability of the light 138 and to provide real time calibration ofthe reflectance measurements made by the imaging sensor 158.Measurements at the reference sensor 134 may be used to adjustcharacteristics of the light sources (e.g., output power) to providespatial and temporal corrections.

FIG. 2 is a simplified cross-sectional view of a multi-wavelength lightsource in accordance with an embodiment. The multi-wavelength lightsource may be used, for example, as part of the source module 102 in theimaging reflectometer 100 of FIG. 1. The multi-wavelength light sourceincludes a plurality of light sources 202 and a plurality of opticalfibers 206. The light sources 202 may each include one or more lightemitting diodes (LEDs) and/or laser diodes (LDs). The light sources 202are each optically coupled with a homogenizer 208 by one of the opticalfibers 206. Each of the light sources 202 generates a beam, and at leastsome of the beams may have different nominal wavelengths. Light from thehomogenizer 208 may be directed to a large field lens and used to imagea sample as described with regard to FIG. 1.

In an embodiment, the multi-wavelength light source sequentiallygenerates different ones of the input beams and/or sequentiallygenerates combinations of multiple input beams. The beams may besequentially generated at a switching rate that is generally the same asthe frame rate of an imaging sensor (e.g., the imaging sensor 158 shownin FIG. 1) to achieve one image per wavelength of the same field on asample. The frame rate of a sensor can be faster than the wavelengthswitching rate in some embodiments. A faster switching rate enablesaveraging of multiple images at each wavelength to achieve higher signalto noise ratio. Output power for each of the light sources 202 may beindependently controlled and adjusted so that the sensor signal is closeto saturation at each wavelength to maximize signal to noise ratio. Eachof the light sources 202 may have sufficient output power to enable highspeed measurements (or measurements at or near a readout speed of theimaging sensor).

In some embodiments, the optical throughput may be increased byinserting diffusers between the optical fibers 206 and the homogenizer208. Multiple light sources 202 can be combined by other means such asdichroic beam splitters, and the light sources 202 can be coupled intothe homogenizer 208 by other means such as free space optics relay.

In some embodiments, band pass filters can be inserted between each ofthe light sources 202 and their respective optical fiber 206 to narrowthe bandwidth of each wavelength. Narrower bandwidths can provide bettersensitivity for measurement of thick film stacks or dense patterns on asurface of the sample. Bandpass filters can define measurementwavelengths accurately by eliminating the wavelength drift of LEDs toimprove measurement accuracy.

The imaging sensor (e.g., the imaging sensor 158 shown in FIG. 1) mayhave a high readout speed (e.g., 50 to 1000 frames per second (FPS) ormore and up to 100 million pixels per frame or more). As an example, ata readout speed of 100 FPS, the imaging sensor may be capable ofperforming 6000 reflectivity measurements per minute. The measurementscan be at the same or different wavelengths. Obtaining multiplemeasurements at the same wavelength can enhance signal to noise ratioand improve measurement sensitivity.

FIG. 3 is a simplified cross-sectional view of an imaging reflectometersystem 300 configured to provide area reflectometry measurements andspot reflectometry measurements in accordance with an embodiment. Inthis example, light from a source module 302 passes through anillumination pupil 314 and is directed toward a large field lens 326.The large field lens 326 may have a field size (or illumination area)that enables area reflectometry measurements without scanning the lightor moving the stage 332. The large field lens 326 may be a telecentriclens so that light rays traveling from the large field lens 326 to asample are substantially parallel to an optical axis and have a lowtelecentricity error similar to the imaging reflectometer of FIG. 1described above.

In this example, the imaging reflectometer system 300 also includes aspot reflectometer 376. The spot reflectometer 376 may be a highsensitivity reflectometer that enables spot reflectometry measurements.The spot reflectometer 376 may be mounted on a robotic arm 372 thatallows movement of the spot reflectometer 376 to any position over asample for spot measurements and/or movement outside a field of view ofthe large field lens 326 during area measurements. The robotic arm maybe, for example, an R-theta robotic arm. Alternatively, the stage 332may be an x-y scanning stage that positions a sample under the largefield lens 326 or the spot reflectometer 376.

In some embodiments, the large field lens 326 may be used to performfull-sample or large area image reflectometry measurements. Using thearea measurements, a particular spot or spots on the sample may beidentified for further measurements, and the spot reflectometer 376 maybe used to perform spot reflectometry measurements at the particularspots. A wavelength range of the spot reflectometer 376 may be greaterthan a wavelength range of the large field lens 326.

FIG. 3 is a simplified cross-sectional view of the imaging reflectometersystem 300, and many parts and components are not shown for simplicity.For example, this figure does not separately show a beam splitter, areference sensor, an imaging sensor, an imaging pupil, and/or a numberof other components. It should be appreciated that the imagingreflectometer system 300 may include these and other components such asthose described with regard to FIG. 1 and/or other conventionalreflectometer systems.

FIG. 4 is a flowchart outlining a method for measuring reflectivity of asample using an imaging reflectometer that includes a large field lensin accordance with an embodiment. The method includes sequentiallygenerating a plurality of input beams at a first switching rate (402).In some embodiments, each of the plurality of input beams are generatedby a different light source, and at least some of the plurality of inputbeams may have different nominal wavelengths than others of theplurality of input beams. In other embodiments, at least some of theplurality of input beams are generated by a broadband light source, anda wavelength of each of the plurality of input beams is defined using aset of band pass filters.

Each of the plurality of input beams is directed through an illuminationpupil having a first NA (404). The illumination pupil may be arrangedalong a first optical path. In some embodiments, each of the pluralityof input beams may be split, and a first portion of each of theplurality of input beams may be directed along a first optical pathtoward a reference sensor, and a second portion of each of the pluralityof input beams may be allowed to continue along the first optical path.

At least a portion of each of the plurality of input beams is providedas substantially telecentric illumination over a sample being imagedusing the large field lens (406). A portion of each of the plurality ofinput beams may also be provided to the reference sensor for monitoringuniformity and stability of the input beams. In some embodiments, ameasurement field of the large field lens may larger than the samplebeing imaged to provide full-sample measurements.

Reflected portions of the substantially telecentric illuminationreflected from the sample are received at the large field lens anddirected through an imaging pupil having a second NA that is lower thanthe first NA of the illumination pupil (408). The reflected portions maybe directed through the imaging pupil using a beam splitter.

The reflected portions are received and corresponding image informationis generated at an imaging sensor module, wherein the image informationis generated at a frame rate that is the same as or faster than thefirst switching rate (410). The image information may be calibrated ornormalized based on information from the reference sensor.

In some embodiments, images obtained using embodiments described hereinmay be processed to identify process excursions. The images may beprocessed in accordance with known excursion identification techniques.For example, the reflectance measured at multiple wavelengths can becompared to modeled reflectance or known good samples. Measured patternsmay also be compared at different locations across the sample toidentify variation and/or outliers. Measured variation can be quantifiedby calculating root-mean-squared (RMS) difference at multiplewavelengths. Measurement sensitivity can be enhanced by selecting thewavelength or wavelengths that have the highest sensitivity based onmeasurement data. Multiple wavelength reflectance can be processed bynon-linear regression to a theoretical model to derive film thicknessand/or CD of a pattern.

It should be appreciated that the imaging reflectometers describedherein may be configured as standalone metrology tools or integratedwith other metrology or process tools. As an example, an imagingreflectometer as described herein may be integrated with a process tooland arranged outside a window separating the imaging reflectometer froma process chamber. In some embodiments, a large field lens arrangedoutside the window provides illumination for a sample arranged insidethe process chamber. The large field lens may be configured to provideillumination to all or a portion of the sample (e.g., a measurement areamay be about the same size as a die or stepper field). This allowsreflectometry measurements to be performed during and/or immediatelyafter processing while samples are inside a vacuum chamber. This canshorten the control loop, improve process control, and avoid materialdamage caused by air environment.

FIGS. 5A-5B illustrate how signal to noise ratio can be increased bycombining measurements from different pixels in an image in accordancewith an embodiment. In FIG. 5A, a measurement area 504 within an image502 is outlined by a square. For purposes of this example, it is assumedthat the measurement area 504 corresponds to a 200 pixel by 200 pixelarea of the image 502 (or of an imaging sensor). A spot 506 within themeasurement area 504 is indicated by an arrow, and the spot 506corresponds to a single pixel.

In an experiment, approximately 100 images were captured, and withineach image a reflectance intensity value from the spot 506 was comparedwith an average reflectance intensity value from the 40,000 pixelswithin the 200 pixel by 200 pixel measurement area 504. The results areshown in FIG. 5B, where the measured reflectance intensity value 508from the single spot 506 has considerably more noise than the averagereflectance intensity value 510 from the 40,000 pixels within themeasurement area 504.

FIG. 6 is an image highlighting similar structures within a measurementarea 604 in accordance with an embodiment. In this image, some of thestructures on a surface of the sample have similar features. Asexamples, the structures within sub-areas 620 a, 620 b, 620 c, 620 d,620 e each have similar structures. Sub-area 620 a is a singlecontiguous area that includes many small structures, some of whichappear to have slightly different patterns, but the small structures areall grouped together within the same part of the measurement area 604.Sub-areas 620 b include four adjacent but separate sub-areas that extendover larger structures that do not appear to be patterned, whilesub-area 620 e include 3 areas that are not adjacent and do not appearto be patterned. Sub-areas 620 b and 620 c also have similar structures.

Signal to noise ratio of reflectance intensity values can be increasedby combining measurements from the pixels within each sub area. Forexample, a representative reflectance intensity value can be determinedfrom the reflectance intensity values of the pixels within sub-area 620a. A parameter associated with the similar structures in sub-area 620 acan be determined using the representative reflectance intensity value.The representative reflectance intensity value may be an average, sum,or the like, and the parameter may be a CD or film thickness. The sameprocess can be performed using the reflectance intensity values of thepixels within each of the other sub-areas. Parameters associated witheach of the other sub-areas 620 b, 620 c, 620 d, 620 e may be determinedin a similar manner.

FIG. 7 shows multiple images 702 a, 702 b, . . . 702 n that each includea measurement area 704 in accordance with an embodiment. In thisexample, the x- and y-axes represent two dimensional locations withinthe images 702 a, 702 b, . . . 702 n, and the z-axis represents time.Although not shown in this example because of the way the images 702 a,702 b, . . . 702 n are stacked, the measurement area 704 is located inthe same area in each image (or the measurement area 704 includes thesame x, y pixels in each image). Also, the measurement area 704 in eachimage includes the same features on the sample.

As explained with regard to FIG. 5B, signal to noise ratio can beincreased by determining an average reflectance intensity value fromeach of the pixels within the measurement area. In a similar manner, thesignal to noise ratio can be increased by using the multiple images 702a, 702 b, . . . 702 n to determine a first average reflectance intensityvalue for each x, y pixel location. That is, a pixel in each image atthe same x, y location can be used to determine a first averagereflectance intensity value for each x, y pixel location in themeasurement area 704. Then, using the first average reflectanceintensity value for each pixel, a second average reflectance intensityvalue can be determined for a given structure within the measurementarea 704 using just those pixels that are associated with the structure.A parameter associated with the structure, such as CD or film thickness,may be determined using the second average reflectance intensity value.

It should be appreciated that while average reflectance intensity valueshave been used in the previous example, other parameters or statisticalmethods may be used in determining representative reflectance intensityvalues. For example, median reflectance intensity values, a sum of thereflectance intensity values, or other parameters or statistical methodsmay be used with the embodiments described herein.

Also, while a pixel in each image at the same x, y location can be usedto determine a first average reflectance intensity value for each x, ypixel location, pixels in each image at approximately the same x, ylocation may be used in some embodiments. Pixels at approximately thesame x, y location include pixels that are associated with approximatelythe same part of the measurement area in each image (i.e., pixels thatare not at the same x, y location but include image information from thesame part of the sample).

FIG. 8 is a flowchart illustrating a method for performing imagingreflectometry measurements in accordance with an embodiment. The methodincludes illuminating a measurement area on a sample using an input beam(802), and receiving portions of the input beam reflected from thesample at an imaging sensor (804). The measurement area may be an areathat is smaller than the sample. The input beam may be a broadband beamor a beam having a narrow wavelength range (e.g., a range of about 0.1nm or less to about 50 nm or more). The imaging sensor may include oneor more digital cameras.

The method also includes obtaining multiple images of the measurementarea using the portions of the input beam reflected from the sample andreceived at the imaging sensor, each of the multiple images comprising aplurality of pixels, wherein corresponding pixels include a single pixelfrom each of the multiple images that is associated with approximately asame part of the measurement area in each of the multiple images (806).The corresponding pixels may each have the same x, y pixel location, orthe corresponding pixels may include image information from the samepart of the sample.

A reflectance intensity value for each of the plurality of pixels ineach of the multiple images is determined (808), and a representativereflectance intensity value is determined for each of the correspondingpixels based on the reflectance intensity value of each pixel of thecorresponding pixels (810). The representative reflectance intensityvalue for each of the corresponding pixels may be, for example, anaverage or sum based on the reflectance intensity value of each pixel ofthe corresponding pixels.

The method also includes determining a parameter associated with astructure on a surface of the sample within the measurement area basedat least in part on the representative reflectance intensity value ofeach of the corresponding pixels that are associated with the structurein the multiple images (812). The parameter associated with thestructure may include a CD and/or film thickness.

FIG. 9 is a flowchart illustrating a method for performing imagingreflectometry measurements in accordance with another embodiment. Themethod includes illuminating a measurement area on a sample using aninput beam, the measurement area including first sub-areas that eachinclude similar structures on a surface of the sample, and secondsub-areas that each include similar structures on a surface of thesample that are different than the structures in the first sub-areas(902). The first and second sub-areas are areas that are smaller (orhave fewer pixels) than the measurement area. The first and secondsub-areas may each include a single area, a single area with structureshaving similar patterns, multiple areas that each have structures havingsimilar patterns, or multiple areas that each have similar structures.The multiple areas may be adjacent or separated from each other with themeasurement area.

The method also includes receiving portions of the input beam reflectedfrom the sample at an imaging sensor (904), and obtaining an image ofthe measurement area using the imaging sensor, the image comprising aplurality of pixels (906).

The method also includes identifying a first portion of the plurality ofpixels that are associated with the first sub-areas and a second portionof the plurality of pixels that are associated with the second sub-areas(908). The pixels that are associated with the first and secondsub-areas may be identified based on reflectance intensity values. Forexample, similar reflectance intensity values may be grouped within asub-area. Alternatively, the pixels that are associated with the firstand second sub-areas may be identified based on x, y locations withinthe image.

A first reflectance intensity value is determined for each pixel of thefirst portion of the plurality of pixels that are associated with thefirst sub-areas (910), a first representative reflectance intensityvalue is determined based on the first reflectance intensity value ofeach pixel in the first portion of the plurality of pixels that areassociated with the first sub-areas (912), and a parameter associatedwith the similar structures in the first sub-areas is determined basedat least in part on the first representative reflectance intensity value(914).

It should be appreciated that the specific steps illustrated in FIGS.8-9 provide particular methods for measuring reflectivity according tosome embodiments. Other sequences of steps may also be performedaccording to alternative embodiments. For example, alternativeembodiments may perform the steps outlined above in a different order.Moreover, the individual steps illustrated in FIGS. 8-9 may includemultiple sub-steps that may be performed in various sequences.Furthermore, additional steps may be added or removed depending on theparticular application.

While the foregoing is directed to specific embodiments, other andfurther embodiments may be devised without departing from the basicscope thereof. For example, features of one or more embodiments of theinvention may be combined with one or more features of other embodimentswithout departing from the scope of the invention. The specification anddrawings are, accordingly, to be regarded in an illustrative rather thana restrictive sense. Thus, the scope of the present invention should bedetermined not with reference to the above description, but should bedetermined with reference to the appended claims along with their fullscope of equivalents.

What is claimed is:
 1. A method for performing imaging reflectometrymeasurements, the method comprising: illuminating a measurement area ona sample using an input beam; receiving portions of the input beamreflected from the sample at an imaging sensor; obtaining multipleimages of the measurement area using the portions of the input beamreflected from the sample and received at the imaging sensor, each ofthe multiple images comprising a plurality of pixels, whereincorresponding pixels include a single pixel from each of the multipleimages that is associated with approximately a same part of themeasurement area in each of the multiple images; determining areflectance intensity value for each of the plurality of pixels in eachof the multiple images; determining a representative reflectanceintensity value for each of the corresponding pixels based on thereflectance intensity value of each pixel of the corresponding pixels;and determining a parameter associated with a structure on a surface ofthe sample within the measurement area based at least in part on therepresentative reflectance intensity value of each of the correspondingpixels that are associated with the structure in the multiple images. 2.The method of claim 1 wherein the representative reflectance intensityvalue for each of the corresponding pixels is an average based on thereflectance intensity value of each pixel of the corresponding pixels.3. The method of claim 1 further comprising determining a compositeimage of the measurement area, wherein each pixel of the composite imageis determined based on the representative reflectance intensity valuefor each of the corresponding pixels.
 4. The method of claim 1 whereinthe parameter associated with the structure is film thickness.
 5. Themethod of claim 1 wherein the parameter associated with the structure iscritical dimension (CD).
 6. The method of claim 1 wherein each of themultiple images are obtained using approximately a same integrationlength of time.
 7. The method of claim 1 wherein each of the multipleimages are obtained using a different integration length of time.
 8. Themethod of claim 1 wherein the portions of the input beam are reflectedfrom the surface of the sample.
 9. The method of claim 1 wherein thestructure comprises a plurality of structures.
 10. A method forperforming imaging reflectometry measurements, the method comprising:illuminating a measurement area on a sample using an input beam, themeasurement area including first sub-areas that each include similarstructures on a surface of the sample, and second sub-areas that eachinclude similar structures on a surface of the sample that are differentthan the similar structures in the first sub-areas; receiving portionsof the input beam reflected from the sample at an imaging sensor;obtaining an image of the measurement area using the imaging sensor, theimage comprising a plurality of pixels; identifying a first portion ofthe plurality of pixels that are associated with the first sub-areas anda second portion of the plurality of pixels that are associated with thesecond sub-areas; determining a first reflectance intensity value foreach pixel of the first portion of the plurality of pixels that areassociated with the first sub-areas; determining a first representativereflectance intensity value based on the first reflectance intensityvalue of each pixel in the first portion of the plurality of pixels thatare associated with the first sub-areas; and determining a parameterassociated with the similar structures in the first sub-areas based atleast in part on the first representative reflectance intensity value.11. The method of claim 10 wherein the first representative reflectanceintensity value is an average based on the first reflectance intensityvalue of each pixel in the first portion of the plurality of pixels. 12.The method of claim 10 wherein at least some of the first sub-areas arenot immediately adjacent to others of the first sub-areas within themeasurement area.
 13. The method of claim 10 wherein the parameterassociated with the similar structures in the first sub-areas is filmthickness.
 14. The method of claim 10 wherein the parameter associatedwith the similar structures in the first sub-areas is critical dimension(CD).
 15. The method of claim 10 wherein the similar structures in thefirst sub-areas have a similar surface pattern, and the similarstructures in the second sub-areas have a different surface pattern. 16.The method of claim 10 further comprising: determining a secondreflectance intensity value for each pixel of the second portion of theplurality of pixels that are associated with the second sub-areas;determining a second representative reflectance intensity value based onthe second reflectance intensity value of each pixel in the secondportion of the plurality of pixels that are associated with the secondsub-areas; and determining a parameter associated with the similarstructures in the second sub-areas based on the second representativereflectance intensity value.
 17. The method of claim 10 wherein thesimilar structures in the first sub-areas each comprise a plurality ofstructures.